What is the Difference Between var, let, and const in JavaScript?

In JavaScript, we declare variables using var, let, or const. While they all store values, they behave differently in terms of scope, hoisting, and mutability.

var – The Old Way

var is the original way to declare variables in JavaScript. It is function-scoped, which means it’s accessible anywhere inside the function it’s declared in. If declared outside of a function, it’s globally scoped.

function example() {
  var message = "Hello";
  if (true) {
    var message = "Hi"; // Same variable!
    console.log(message); // "Hi"
  }
  console.log(message); // "Hi"
}

Note: The variable got overwritten inside the if block. That’s because var doesn’t care about block scope.

let – The Modern and Safer Option

let was introduced in ES6 (2015). It is block-scoped, meaning it’s only accessible within the block where it’s defined.

function example() {
  let message = "Hello";
  if (true) {
    let message = "Hi"; // Different variable!
    console.log(message); // "Hi"
  }
  console.log(message); // "Hello"
}

This makes let much safer and predictable for most use cases.

const – For Constants

const is also block-scoped like let, but it creates a read-only reference to a value. You cannot reassign it.

const name = "Alice";
name = "Bob"; // ❌ Error: Assignment to constant variable

However, const doesn’t make the value itself immutable if it’s an object or array:

const user = { name: "Alice" };
user.name = "Bob"; // ✅ Works!

Hoisting Differences

• var is hoisted and initialized as undefined.
• let and const are hoisted but not initialized. Accessing them before declaration results in a ReferenceError.

console.log(a); // undefined
var a = 10;

console.log(b); // ReferenceError
let b = 20;

When to Use What?

• Use const by default for values that shouldn’t change.
• Use let when you expect the value to change.
• Avoid var in modern code unless working with legacy projects.